Semiconductor devices which are semiconductor apparatuses, particularly silicon devices have undergone high integration and low electrical power consumption by miniaturization at a rate of quadrupling every three years according to a scaling law called Moore's law. In recent years, the gate length of MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) has shrunk down to 20 nm or less. As a result, elevated prices of lithography processes, i.e., those of lithography apparatuses and mask sets, and physical limits owing to the dimension of the devices, i.e., the operation limit and variation limit thereof have made impossible the scaling at the existing rate. Accordingly, device performance is required to be improved by an approach different from the scaling law.
In recent years, using back-end devices has been expected to be an improvement method independent of the scaling law. The back-end devices are semiconductor devices including an element provided in a multilayered wiring layer of a semiconductor apparatus, such as an element which changes its resistance in a non-volatile fashion. Examples of the non-volatile resistance-changing element include a resistance-changing element used for MRAMs (Magneto-resistive Random Access Memories), PRAMs (Phase-change Random Access Memories), ReRAMs (Resistance Random Access Memories), and so on.
When used as memories or switches provided in a multilayered wiring layer of CMOS (Complementary Metal Oxide Semiconductor) semiconductor devices, these resistance-changing elements are expected to lower the power consumption of semiconductor devices. Further, these resistance-changing elements are expected to increase mounting capacity in association with the trend of the miniaturization of semiconductor devices and increase in data storage capacity.
On the other hands, in recent years, a rewritable programmable logic device called FPGA (Field-Programmable Gate Array) has been developed, which is regarded as an intermediate device between the gate-array and the standard cell. The FPGA enables customers themselves to switch the circuit architecture of the post-manufactured chip. The resistance-changing element provided in a multilayered wiring layer is expected to perform such switching of the circuit architecture. The reason is that the FPGA configured by using a resistance-changing element allows lowering power consumption, while improving the degree of freedom of the circuit architecture.
Examples of the preferable resistance-changing element for the application to such switching of the circuit architecture in the FPGA include NanoBridge® exploiting an ion-conductor, which is one form of ReRAMs. The ion-conductor is a solid electrolyte in which ions can be freely moved by an applied field such as electric field.
PTL 1 and NPL 1 disclose switching elements (also referred to as solid electrolyte switches) exploiting filament formation by both metal ion transfer and an electro-chemical reaction in the ion-conductor. The switching elements disclosed in PTL 1 and NPL 1 have not only an ion-conducting layer but also a first electrode (activating electrode) and a second electrode (deactivating electrode) which are disposed opposite to each other across the ion-conducting layer. Among them, the first electrode plays a role of supplying metal ions to the ion-conducting layer. The second electrode does not supply metal ions to the ion-conducting layer.
The operation of the switching element will be explained briefly. Earthing the first electrode and applying a negative voltage to the second electrode generates metal ions from the metal of the first electrode, and the ions dissolve into the ion-conducting layer. Then, the metal ions in the ion-conducting layer segregate as metal into the ion-conducting layer, and the segregated metal forms a metal cross-linking (filament) which connects the first electrode and second electrode. The switch is put in ON-state by the electrical connection of the first electrode and the second electrode owing to the metal cross-linking.
On the other hands, earthing the first electrode and applying a positive voltage to the second electrode in ON-state cuts a portion of the metal cross-linking. Thus, the electrical connection of the first electrode to the second electrode is cut to put the switch in OFF-state. An electrical property, such as increase in the resistance across the first electrode and the second electrode and variation in the inter-electrode capacity, begins changing before the complete electrical disconnection, and eventually leads to the electrical disconnection. Putting the switch in ON-state from OFF-state may be carried out again by earthing the first electrode and applying a negative voltage to the second electrode.
Such a switching element is characterized in that its size and on-resistance are smaller than those of semiconductor switches such as MOSFETs. Accordingly, the switching element is thought to be promising for application to programmable logic devices such as FPGAs. Further, in this switching element, since the electrically connected state, i.e., ON-state or OFF-state of the element is kept as is even without applying voltage, it is also possible to apply the element to a non-volatile memory element.
For example, a memory cell is prepared which is an elementary unit including one selecting element such as transistor and one switching element, and a plurality of the memory cells are disposed both in one direction and in another direction perpendicular thereto. Such an arrangement enables selecting any memory cell among a plurality of the memory cells by using a word-line and a bit-line. Thus, a non-volatile memory can be actualized which enables sensing the electrically connected state of a switching element in a selected memory cell, to read information of either “1” or “0” from ON- or OFF-state of the switching element.